The present invention is related to an improved method for preparing solid electrolytic capacitors with high temperature reliability.
The present invention is related to an improved method of forming a solid electrolyte capacitor and an improved capacitor formed thereby. More specifically, the present invention is related to a capacitor with improved long term leakage performance at 200° C. and above.
The construction and manufacture of solid electrolyte capacitors is well documented. In the construction of a solid electrolytic capacitor a valve metal serves as the anode. The anode body can be either a porous pellet, formed by pressing and sintering a high purity powder, or a foil which is etched to provide an increased anode surface area. An oxide of the valve metal is electrolytically formed to cover all surfaces of the anode and to serve as the dielectric of the capacitor. The solid cathode electrolyte is typically chosen from a very limited class of materials, to include manganese dioxide or electrically conductive organic materials such as 7,7,8,8 tetracyanoquinonedimethane (TCNQ) complex salt, or intrinsically conductive polymers, such as polyaniline, polypyrol, polythiophene and their derivatives. The solid cathode electrolyte is applied so that it covers the dielectric surfaces and is in direct intimate contact with the dielectric. In addition to the solid electrolyte, the cathodic layer of a solid electrolyte capacitor typically consists of several layers which are external to the anode body. In the case of surface mount constructions these layers typically include: a carbon layer; a cathode conductive layer which may be a layer containing a highly conductive metal, typically silver, bound in a polymer or resin matrix; and a conductive adhesive layer such as silver filled adhesive. The layers including the solid cathode electrolyte, conductive adhesive and layers there between are referred to collectively herein as the cathode layer which typically includes multiple layers designed to allow adhesion on one face to the dielectric and on the other face to the cathode lead. A highly conductive metal lead frame is used as a cathode lead for negative termination. The various layers connect the solid electrolyte to the outside circuit and also serves to protect the dielectric from thermo-mechanical damage that may occur during subsequent processing, board mounting, or customer use.
The cathodic conductive layer, which is typically a silver layer, serves to conduct current from the lead frame to the cathode and around the cathode to the sides not directly connected to the lead frame. The critical characteristics of this layer are high conductivity, adhesive strength to the carbon layer, wetting of the carbon layer, and acceptable mechanical properties.
The oldest, and currently largest, user of high-temperature electronics (>150° C.) is the downhole oil and gas industry (Analog Dialogue 46-04, April 2012). In this application, the operating temperature is a function of the underground depth of the well. Worldwide, the typical geothermal gradient is 25° C./km depth, but in some areas, it is greater. In the past, drilling operations have maxed out at temperatures of 150° C. to 175° C., but declining reserves of easily accessible natural resources coupled with advances in technology have motivated the industry to drill deeper, as well as in regions of the world with a higher geothermal gradient. Temperatures in these hostile wells can exceed 200° C., with pressures greater than 25 kpsi. Active cooling is not practical in this harsh environment, and passive cooling techniques are not effective when the heating is not confined to the electronics. Besides the oil and gas industries, other applications, such as avionics, are emerging for high-temperature electronics.
U.S. Pat. No. 7,233,483, which is incorporated herein by reference, teaches a method for improving high temperature (85° C.) performance of capacitors wherein the cathode comprises a silver layer with the silver layer further comprising silver and/or sulfur compounds. The performance is inadequate for higher temperature applications.
US 2012/0106031, which is incorporated herein by reference, claims an improved capacitor assembly for use in high voltage and high temperature environments wherein the capacitor element is enclosed and hermetically sealed within a housing in the presence of a gaseous atmosphere that contains an inert gas. It is believed that the housing and inert gas atmosphere are capable of limiting the amount of oxygen and moisture supplied to the conductive polymer of the capacitor. In this manner, the solid electrolyte is less likely to undergo a reaction in high temperature environments, thus increasing the thermal stability of the capacitor assembly. Though capable of functioning at temperatures of about 215° C. or 230° C. this requires the capacitor to be in an environment which is void of moisture and air which is at least impractical if not impossible under typical working environments.
The prior art methods do not offer a solution for leakage degradation at high temperature such as 200° C. or above typically seen in an oil rig environment.
Thus there is a need for a solid electrolytic capacitor, and a method of making a solid electrolytic capacitor, which has good reliability when exposed to a temperature of 200° C. or above for 1000 hrs or the duration of intended application. A particular need is for a capacitor with a stable leakage and ESR at 200° C. or above.